1. Field of the Invention
The present invention relates to a waste water treatment process and apparatus, and in particular, to a waste water treatment process and apparatus that ozonizes a microorganism-mixed liquid during a waste water treatment process such as an activated-sludge process using microorganisms in order to reduce the amount of microorganisms occurring, to prevent a microorganism aggregate, that is, a microorganism floc from sedimenting inappropriately, and to recover the inappropriate sedimentation while maintaining the quality of treated water.
2. Description of Related Art
Those treatment processes such as activated-sludge processes which use microorganisms and which are conventionally and commonly used to treat waste water may cause a large amount of sludge to remain due to the waste water treatment. The excess sludge is mostly disposed of by incineration after dehydration and is sometimes digested by microorganisms. The disposal of the excess sludge, however, requires a large amount of energy and costs. Thus, it is necessary to minimize the sludge generated after waste water treatment.
In addition, during waste water treatment, the multiplication of filamentous bacteria may prevent the sludge from sedimenting appropriately, often making solid-liquid separation in a precipitator insufficient. In this case, microorganisms may flow out with the treated water to degrade its quality. Thus, to provide stable waste water treatment, the inappropriate sedimentation of the sludge must be prevented and the appropriate sedimentation of the sludge must be recovered.
As a process for reducing the sludge generated after waste water treatment, for example, a process for drawing activated sludge from a treatment system and introducing the sludge into the treatment system after ozonization is proposed in Japanese Patent Laid-Open No. 6-206088. FIG. 26 shows an example of treatment flow for a conventional waste water ozonization process. As shown in the figure, an incoming water channel 2 is connected to an inlet side of an aeration vessel 1, and an interconnecting channel 3 is connected to an outlet side of the vessel 1. In addition, a drawn-sludge channel 6 including a sludge-drawing pump 5 in its middle is introduced into an ozonization vessel 4 from the aeration vessel 1. The ozonization vessel 4 connects to an ozone supply channel 8 that supplies ozone; an ozone discharge channel 7 that discharges ozone that has not been used for reaction with sludge; and an ozonized sludge channel 9 that returns ozonized sludge to the aeration vessel 1. A diffuser is provided on the bottom of the aeration vessel 1 and an air supply channel is connected to the diffuser. The interconnecting channel 3 is further connected to a sludge separation section to which treated-liquid channel and a sludge-drawn channel are connected, and a returned-sludge channel including a sludge-returning pump connects a sludge draw-out channel and the aeration vessel 1 together. The description of these components, however, are omitted.
Next, a conventional treatment process is described. Organic waste water is introduced into the aeration vessel 1 through the incoming water channel 2, and air is then diffused in the aeration vessel 1 to activate sludge for aerobic treatment. Then, the mixed-liquid in the aeration vessel 1 is delivered to the sludge separation section through the interconnecting channel 3 for solid-liquid separation, and the separated-liquid is discharged as treated water. Sludge drawn by the sludge-drawing pump 5 from the aeration vessel 1 through the drawn-sludge channel 6 is circulated to the ozonization vessel 4 where the sludge is contacted with ozone provided through the ozone supply channel 8 and subjected to ozonization. The ozonized sludge is returned to the aeration vessel 1 through the ozonized-sludge channel 9 for aerobic treatment.
In this process, activated sludge the amount of which is larger than the amount of sludge multiplied by the assimilation of BOD in the treated-liquid is drawn from an aerobic treatment system, ozonized, and introduced into the aerobic treatment system to reduce the occurrence of excess sludge.
For example, Yasui, et al. reported on the ozonization of activated ozone drawn from the treatment system (Wat. Sci. Tech., 30, 11 (1994)). FIG. 27 is a characteristic diagram showing the relationship between ozone dose and the concentrations of MLVSS and soluble TOC in a case in which 30 mg/L of ozone is used for drawn activated sludge to execute batch ozonization.
This figure indicates that the ozonization of activated sludge does not substantially change the concentration of MLVSS and slightly increases the concentration of soluble TOC. Under these ozonization conditions, only part of the activated sludge is solubilized and little activated sludge is made inorganic.
During ozonization, the conventional techniques continuously inject ozone into the activated sludge drawn from the aeration vessel, so a large ozone dose must be injected to reduce the amount of excess sludge, thereby increasing ozone manufacturing costs.
In addition, under the ozonization conditions in FIG. 27, the ozonization solubilizes only part of the sludge and does not make it inorganic, resulting in few changes in the amount of organic substance. Thus, when such ozonization is carried out during the conventional treatment process that ozonizes the drawn activated-sludge before returning it to the aeration vessel, all organic substance corresponding to the drawn sludge act as loads on the aeration vessel to increase the load, thereby increasing the concentration of organic substance in treated water to degrade its quality. The amount of aeration required to maintain the quality of treated water is also increased.
In addition, since the only effect of the introduction of the ozonization of the activated sludge according to the conventional techniques is the reduction of the amount of excess sludge, these techniques cannot prevent the inappropriate sedimentation of activated sludge that significantly hinders the solid-liquid separation of the sludge or recover appropriate sedimentation.
In addition, the conventional techniques executes ozonization regardless of the organic substances decomposition activity of the activated sludge. Consequently, if an external factor such as a qualitative change in incoming waste water or a change in temperature, or the introduced ozone reduces the organic substances decomposition activity of the activated sludge, the ozonization further significantly reduces the organic substances decomposition activity of the activated sludge to increase the concentration of organic substances in treated water, that is, to degrade the quality of the treated water.
In addition, the conventional techniques carry out ozonization regardless of the variation of incoming organic loads. Consequently, if the concentration of organic substances in the incoming waste water or the amount of water discharged increases to increase the organic loads, the ozonization further significantly increases the organic loads on the aeration vessel to increase the concentration of organic substances in the treated water, thereby degrading the quality of the treated water. It also increases the amount of aeration required to maintain the quality of the treated water.
In addition, the conventional techniques execute ozonization and air supply to the aeration vessel regardless of the concentration of dissolved oxygen in the activated-sludge-mixed liquid in the aeration vessel. Thus, if the organic loads increase, to reduce the amount of dissolved oxygen in the activated-sludge-mixed liquid, the ozonization further reduces the amount of dissolved oxygen to increase the concentration of organic substances in the treated water, thereby degrading the quality of the treated water. In addition, if the organic substances decomposition activity of the activated sludge decreases to increase the amount of dissolved oxygen in the activated-sludge-mixed liquid, the ozonization further reduces the organic substances decomposition activity of the activated sludge to increase the concentration of organic substances in the treated water, thereby degrading the quality of the treated water.
In addition, the conventional techniques carry out the ozonization regardless of the amount and composition of extracellular organic substances in an activated sludge floc that is a microorganism aggregate in the activated sludge. Consequently, if a large amount of extracellular organic substances are contained in the activated sludge floc, most of the injected ozone reacts with the extracellular organic substances in the activated sludge floc and the injected ozone is not effectively used for reaction with activated-sludge microorganisms, thereby preventing the amount of excess sludge resulting from the ozonization from being stably reduced.
In addition, the conventional techniques carry out the ozonization regardless of the resistivity of the activated-sludge microorganisms to ozone. As a result, if anti-oxidization enzymes in the activated-sludge microorganisms that protect the microorganisms are very active and the activated-sludge microorganisms have a high resistance to oxidization, the injected ozone cannot kill the microorganisms and is not effectively used to make the activated-sludge microorganisms inorganic or for reaction for solubilization, thereby preventing the amount of excess sludge resulting from the ozonization from being stably reduced.
In addition, according to the conventional techniques, the effects of the introduction of the ozonization of the activated sludge can be provided only for an aeration vessel consisting of a single vessel but not for an aeration vessel consisting of multiple vessels.
In addition, the conventional techniques do not adjust the amount of ozonized sludge in returning the sludge to the aeration vessel. Thus, if the organic loads temporarily increase due to an increase in the concentration of organic substances in the incoming waste waster or in the amount of water discharged, the ozonization further significantly increases the organism loads relative to the activated sludge in the aeration vessel to increase the concentration of organisms in the treated water, thereby degrading the quality of the treated water. It also increases the amount of aeration required to maintain the quality of the treated water.
In addition, in the ozonization of the activated sludge, the conventional techniques have a low ozone solution efficiency, a low efficiency in the contact between the activated sludge and ozone, and a low reaction efficiency due to their simple injection of ozone into the activated sludge.
In addition, since the conventional techniques use only ozone to treat the drawn activated-sludge without the use of other strong oxidizing agents, the activated-sludge microorganisms are killed but are unlikely to become soluble or inorganic. Consequently, such ozonization causes all organic substances corresponding to the drawn sludge to act as loads on the aeration vessel to increase the load and thus the concentration of organic substances in the treated water, thereby degrading the quality of the treated water. It also increases the amount of aeration required to maintain the quality of the treated water.